Data have now been collected which support the hypothesis that infection with hepatitis B virus (HBV) is required for the development of primary hepatocellular carcinoma (PHC) in a very large numbe of the cases of the disease which occur in areas where PHC and carriers of the HBV are common. Strategies directed toward the prevention of infection with HBV with a subsequent decrease in the frequency of PHC are being designed. The recently discovered relation between woodchuck hepatitis virus and cancer of the liver in Marmota monax is another example of an association between a common virus and a cancer in mammals in which infection in the virus appears to be necessary for the development of the cancer. Attempts to discover othr such viruses are in progress. The objective of this would be to identify other preventable cancers.